Sky & Telescope - March 2023 - 60

Amplifier
and readout
Pixels
Pixels
INTERNAL DIFFERENCES: LEAH TISCIONE / S&T
Tech Evolution
INTERNAL DIFFERENCES Although
CCD and CMOS chips function in largely
the same way, one important difference
is how they read out signal. A CCD (near
right) moves electrons (red arrows) off
the detector in rows. The electrons are
then sent off-chip to the amplifier and
analog-to-digital converter. By contrast,
the CMOS detector (far right) has an
amplifier behind every pixel and an A-to-D
converter for each column.
Light
Light
Amplifier,
A-to-D
Column readout
CCD
contrast, CMOS detectors incorporate an amplifier behind
every pixel and an A-to-D converter for each column. This,
combined with a fast USB 3.0 interface, means large amounts
of data download extremely fast. A good example is the
QHY600M reviewed in our July 2020 issue (page 68) that
downloads a 60-megapixel image in under 5 seconds. On top
of that, the camera includes an internal buffer where data are
stored as they're transferred, allowing the camera to begin
the next exposure even before it completes transferring the
previous one. These improvements can increase your imaging
efficiency. Less time lost to data transfers means more time
for recording photons. In my case, the difference works out to
an additional 5-minute exposure per hour compared to when
I used a USB 2.0 CCD camera.
What This Means for You
Taken together, how does all this affect your future imaging?
Let's look at the possibilities.
If you've considered increasing the resolution of your
conversion
A-to-D conversion
(per column)
CMOS
imaging setup, you might accomplish this by selecting a camera
with smaller pixels than the one you currently use. Pixel
size affects resolution, and since most CMOS sensors have
smaller pixels than those found on CCDs, stepping up to a
new camera can improve your resolution and image scale, but
only to a point.
For example, I image through a Sky-Watcher Esprit 150mm
refractor, which has a focal length of 1,070 millimeters
(42 inches). When I pair this with my SBIG STL-11000M CCD
camera, which has 9-micron pixels, it produces an image scale
of 1.73 arcseconds per pixel. The same scope yields 0.72 arcseconds
per pixel when combined with my QHY600M CMOS
camera and its 3.76-micron pixels. This results in a modest
resolution gain in my system at the cost of a new camera.
You can use the following formula to determine the arcseconds
per pixel of any camera and telescope combination:
Image scale = ( pixel size
focal length ) × 206.265
Pixel size is measured in microns, and the focal length is
measured in millimeters.
CCD
CMOS
THE DEVIL IS IN THE DETAILS These uncalibrated 5-minute exposures of M74 were recorded through the same 6-inch refractor. The right image
was captured with a QHY600M CMOS camera and displays less noise and far fewer hot pixels compared to the left image taken with a QHY16200A
CCD model.
60 MARCH 2023 * SKY & TELESCOPE

Sky & Telescope - March 2023

Table of Contents for the Digital Edition of Sky & Telescope - March 2023